1. Field of the Invention
This invention relates to an optical connector and, in particular, to an optical connector that is capable of easy connecting optical fibers in a setting site thereof. Also, this invention relates to a connection structure of optical fibers.
2. Description of the Related Art
At present, in order to connect optical fibers, a physical connecting method is widely used that is conducted by butting against each other end faces of the optical fibers or end faces of the ferrules which houses the optical fiber therein. The physical connecting method includes a connecting method using a mechanical splice and a connecting method using an optical connector. Generally, in case of permanent connection (i.e., the optical fibers are usually not detached after they are connected), the connecting method using the mechanical splice is suitably used. Different from the permanent connection, in case of frequently detaching the optical fibers, the connecting method using the optical connector is suitably used. Practically, the mechanical splice is used for the permanent connection and the optical connector is used for the case that the optical fibers are frequently detached.
The connecting method using the mechanical splice and the connecting method using the optical connector are both conducted by applying a pressing force along the axis direction of the optical fibers mutually to the end face of the optical fibers. Thus, particularly in the connecting method using the optical connector that the frequent detaching can be simulated, it is necessary to prevent the damage of the end face of the optical fibers. Therefore, the optical fibers are physically contacted each other at the end face thereof while being protected by a ferrule into which they are inserted (See JP-A-1996-114724 (hereinafter called patent document 1)).
In the physical connecting method (such as the connecting method using the mechanical splice and the connecting method using the optical connector), a profile of the end face of optical fiber can seriously affect its connection characteristics. For example, when angle of the end face of the optical fiber is not aligned or the profile of the end face of the optical fiber is rough, air may enter in a space between the end face of the optical fibers to butt and contact each other. As a result, Fresnel reflection increases at the end face of the optical fiber to cause an increase in connection loss of the optical fiber. A method for preventing the increase in connection loss of the optical fiber is known that the end face of the optical fiber or the ferrule is polished after cutting the optical fiber.
Another method is known that the optical fibers are connected each other without polishing the end face of the optical fiber or the ferrule after cutting the optical fiber. This method is conducted such that a refractive index matching agent in liquid or grease state is interposed between the end faces of the optical fibers, where the agent has a refractive index similar to that of a core of the optical fiber (See JP-A-1999-72641 (hereinafter called patent document 2) and JP-A-1999-101919 (hereinafter called patent document 3)). In this method, the refractive index matching agent is coated on the end face of the optical fibers, or filled into a connection portion of the optical fibers, and then the optical fibers are butted each other. Thus, air can be prevented from entering into the space between the end faces of the optical fibers, the Fresnel reflection caused by the air can be lowered, and the connection loss of the optical fiber can be reduced.
Another method is known that uses a refractive index matching film (hereinafter simply called film) (See JP-B-2676705 (hereinafter called patent document 4), JP-A-2001-324641 (hereinafter called patent document 5) and JP-A-1980-153912 (hereinafter called patent document 6)).
However, of the conventional methods, the method of polishing the end face of the optical fibers or the ferrule is not suitable for connecting the optical fibers, especially an optical fiber (hereinafter called holey fiber) having plural periodical hole portions therein. When the optical fibers are connected in a setting site, a lot of time consumption and labor charge are required for polishing the end face of the optical fibers etc., and a polishing device must be prepared for polishing the end face of the optical fibers etc. Further, when the end face of the holey fiber is polished, a polishing residue generated in the polishing process and an abrasive agent used in the polishing process may enter the hole portion of the holey fiber to cause an increase in connection loss and a reduction in reliability.
On the other hand, the method of interposing the refractive index matching agent between the connection ends of the optical fibers as disclosed in the patent documents 2, 3 has the following problem. Since the refractive index matching agent generally is formed of a silicone or paraffin agent in liquid or grease state, it can enter into the hole portions of the holey fiber when the optical fibers are connected each other by using the optical connecter having the refractive index matching agent therein. Further, this method has the problem that, since the refractive index matching agent generally has a temperature dependency in refractive index, transmission loss of the holey fiber is extremely changed according to a change in refractive index of the refractive index matching agent entered into the hole portion of the holey fiber. Furthermore, this method has the problem that, when the refractive index matching agent enters into the hole portion of the holey fiber, the refractive index matching agent between the end faces of the optical fibers decreases so that air gaps and air bubbles are easily generated between the end faces of the optical fibers to cause the significant lowering of optical characteristics of the holey fiber.
A method may be simulated that the hole portions at the end face of the holey fiber are sealed before connecting the holey fiber. However, such a method is not suitable for connecting the optical fibers at the setting site since a dedicated connecting device is needed for performing the method and a lot of time consumption is required for sealing the hole portions at the end face of the holey fiber to cause an increase in connection cost of the optical fibers.
Furthermore, the methods of using the film as disclosed in the patent documents 4 to 6 have the following problem. Since the diameter of the optical fiber is as fine as 80 or 125 μm, it is very hard to attach the film onto the end face of the optical fibers at high dimensional accuracy. Further, the film needs to have an adhesive ability or stickiness in order to attach the film onto the end face of the optical fibers. Therefore, when the optical fibers are connected each other at the setting site, a foreign substance such as a dust can easily adhere to the film to reduce reliability of the optical fiber and workability during the connection process of the optical fibers.